It has become commonplace to distribute software or other information, such as music and films, on optically readable media, such as CDs and DVDs. Successful readout of an optical disk by current disk readers heavily relies on a number of parameters that characterize the readout laser beam on its path from the laser to the reflective data layer of the disk and back to the optical pickup system of the reader. The electromagnetic wave structure of the readout beam is described by intensity, phase, polarization, and wave vectors of the wave components that constitute the readout beam. The wave structure of the beam determines geometrical and propagation parameters of the beam, such as beam size, angle of incidence, and angle of convergence.
In addition to reading the data layer of a disk, the reader performs other functions, which are as critical for playability as the data readout itself. These functions include auto-focusing, auto-tracking and error correction. The first two functions allow the reader device to actively control motion of the laser head and spindle motor in order to maintain the required position of the beam relative to the disk. Therefore, maintaining the integrity of the wave structure of the beam throughout its path inside the disk material is a key not only to the successful detection and decoding of the information carried by the beam, but also for the continuity of the readout process.
It is desirable in many applications to limit the duration of time during which the optical disk can be read. For example, one could rent an optical disk containing a film or a musical composition that, after some period of time, would become unreadable. In this case there would be no need to return the optical disk to place where it was rented.
U.S. Pat. No. 5,815,484 discloses an optical disk having a reflective metallic layer with a plurality of data structures (provided in the form of pits and lands) and a compound that reacts with oxygen that is superimposed over at least some of the data structures for inhibiting reading of the information.
Commonly assigned U.S. Pat. No. 6,011,772 discloses a read inhibit mechanism that may also use a barrier layer. For example, the removal of a barrier layer enhances the action of a reading-inhibit agent to prevent machine reading of information encoding features on the optical disk. The reading-inhibit agent may also be activated by exposure to optical radiation, or by rotation of the disk.
A disadvantage of some chemical moieties used in or proposed for use in a reading inhibit layer is that they exhibit poor lightfastness, which could allow the limited play optical disk to be photobleached and subsequently converted back, possibly permanently, to a readable optical disk.